When a document is read by a scanner, if a size of a document readable by the scanner is smaller than a size of the document to be read, the following reading methods are used. In a first reading method, for example, in the case where a document size readable by the scanner is A4, and a size of the document to be read is A3, the A3-size document is folded in two, is held at a predetermined position using special sheets, then is subjected to double-sided scanning, and scanned images are concatenated. Also, in a second reading method, for example, in the case where a document size readable by the scanner is A4, and a size of the document to be read is A3, the scanner is set to a non-separation-document feed mode, the A3-size document folded in two is subjected to double-sided scanning, and then scanned images are concatenated.
However, in the first reading method and in the second reading method, a user has to fold a document in two, and then manually set the document one by one to be held with the special sheets, or place the document in the scanner. Accordingly, if there is a large volume of document to be read, an auto document feeder (ADF) is not fully utilized, and thus it takes enormous time and effort for manual operation. Accordingly, the following method is considered as a third reading method. In the third reading method, a document is separated into pieces each having a size readable with a scanner (for example, an A4 size, which is a half of an A3 size), the document is scanned using the ADF, and then the read images are reconstructed (concatenated) into an image of the original size. However, in the third reading method, because of fluctuations of document transport speed at reading, or the like, if a plurality of read images are simply concatenated, mismatching may occur at a boundary of images.
In relation to the above, a technique of automatically connecting divided and read images has been proposed. In this technique, line segments that exist in the direction vertical to a boundary direction are extracted, and one of the images is translated and the position thereof is adjusted so that a difference does not arise on the extracted line segments at the boundary of the images.
Also, a technique has been proposed in which for a character and a figure that spread over a first image and a second image, positions of a right image and a left image are adjusted so that matching points become the maximum and a combined image having little misalignment is obtained.
Related-art techniques have been disclosed in Japanese Laid-open Patent Publication Nos. 07-23204 and 08-204945.